MultiversX Cross-Contract Storage Reads

Read another contract's storage mappers directly — zero gas overhead from proxy calls, no async complexity.

When to Use

Core Pattern

#[multiversx_sc::module]
pub trait ExternalStorageModule {
    // Simple value
    #[storage_mapper_from_address("total_supply")]
    fn external_total_supply(&self, contract_address: ManagedAddress) -> SingleValueMapper<BigUint, ManagedAddress>;

    // Composite key (token-specific)
    #[storage_mapper_from_address("balance")]
    fn external_balance(&self, contract_address: ManagedAddress, token_id: &TokenIdentifier) -> SingleValueMapper<BigUint, ManagedAddress>;

    // Module-prefixed key
    #[storage_mapper_from_address("pause_module:paused")]
    fn external_paused(&self, contract_address: ManagedAddress) -> SingleValueMapper<bool, ManagedAddress>;
}

Key Rules

1. String must EXACTLY match storage key in target contract
2. First param must be ManagedAddress
3. Return type's second generic must be ManagedAddress
4. Additional params become composite key parts

How to Discover Storage Keys

1. Source code: #[storage_mapper("key")] in target
2. ABI: .abi.json lists all keys
3. Module keys: prefixed like pause_module:paused

Security

• Same-shard only: Cross-shard reads return defaults silently — no error
• Stale data: Reflects target's state as of current block
• Key changes: If target renames keys in upgrade, reads break silently
• Read-only: Cannot write to another contract's storage

Anti-Patterns

• Not validating empty results (get default 0/false silently)
• Not validating target is on same shard